The present invention generally relates to dynamoelectric machines, such as generators used in the production of electrical power. More particularly, this invention relates to minimizing eddy current heating in a stator caused by magnetic fields in end-turn regions of the stator.
Large turbine-driven generators used in the production of electrical power comprise a rotor that serves as a source of magnetic lines of flux produced by a wound coil carried on the rotor. The rotor rotates within a stator that comprises a number of conductors in which an alternating current is induced by the rotor as it rotates within the stator, generating a rotating magnetic field in a narrow air gap between the stator and rotor.
FIG. 1 represents adjacent end portions of a stator 10 and rotor 12 illustrative of the type used in certain dynamoelectric machines, such as turbine-driven generators used to generate electrical power. The stator 10 has a generally annular shape that circumscribes the rotor 12, which is generally a large cylindrical body from which spindles (not shown) extend for rotatably supporting the rotor 12 within the stator 10. The rotor 12 has a series of longitudinal (axially-extending) slots 30 in its outer circumference, which result in radially-extending teeth being defined along the perimeter of the rotor 12. Field windings 32, each comprising multiple insulated conductor strands, are installed in the slots 30 to extend the length of the rotor 12, longitudinally projecting from each end of the rotor 12. The field windings 32 include end turns 34, each of which electrically connects a winding 32 within one slot 30 to a second winding 32 in an adjacent slot 30. As the rotor 12 spins, the end turns 34 are subjected to centrifugal forces that urge the end turns 34 radially outward. This radial movement of the end turns 34 is confined by retaining rings 36 attached to the ends of the rotor 12 to enclose the end turns 34, as shown in FIG. 1.
The stator 10 comprises sheets (punchings) 14 supported in a frame 16 so as to be perpendicular to the common axis of the stator 10 and rotor 12. The sheets 14 are formed of a low loss, low magnetic reluctance material, such as a silicon steel, and compressed against each other in bundles 18, which are axially separated by air gaps 20 maintained by nonmagnetic spacers (not shown) between the sheet bundles 18. Armature windings 24 are positioned in slots (not shown) formed in the sheets 14, and end turns 26 of the windings 24 extend outward from the stator 10 around the rotor retaining ring 36. The extent to which the windings 24 extend beyond the end of the stator 10 is reduced by forming the windings 24 as involutes oriented at an angle to the longitudinal axis of the machine, as represented in FIG. 1.
The sheets 14 of the stator 10 are axially compressed by annular-shaped flanges 22, one of which is shown in FIG. 1. The flanges 22 must have adequate strength to support and maintain the positions of the sheets 14 within the stator 10, and therefore must be formed of high strength material. A common example is ductile iron (cast nodular iron) alloys due to their strength, toughness, and machinability. As a particular example, ASTM A536 GR 60-40-18 ductile iron has been used to form stator flanges in generators produced by the General Electric Company. The alloy composition per the ASTM A536 specification is generic in nature, subordinates chemical composition to mechanical properties, and is not optimized for electrical or magnetic permeability properties. As such, components formed of ASTM A536 are mainly chosen to meet mechanical properties and obtain a spheroidal graphite microstructure with a predominantly ferritic matrix. A typical commercial grade of the ASTM A536 alloy contains, by weight, at least 3.0% carbon, at least 1.7% silicon, at least 0.03% magnesium, less than 0.1% phosphorus, less than 0.025% sulfur, the balance iron and incidental impurities.
In a stator 10 having the construction described above, magnetic flux is generated by the end turns 26 and directed parallel to the longitudinal axis of the machine toward the major surfaces of the sheets 14. This magnetic flux induces large eddy currents in the sheets 14 that cause a significant amount of joule (ohmic) heating in the sheets 14, and consequently heating of the stator flanges 22. The alternating magnetic fields of the stator 10 also induce eddy currents in the stator flanges 22, resulting in further heating of the flanges 22. In addition to energy losses that reduce the efficiency of the machine, heating of the sheets 14 and flanges 22 in the vicinity of the stator ends can be sufficient to cause local overheating that is detrimental to the operation of the machine.
For this reason, the stator 10 is shown equipped with an annular-shaped flux shield 28 located adjacent the flange 22 and secured by, for example, straps (as shown), fasteners, etc. Examples of flux shields include U.S. Pat. No. 1,677,004 to Pohl and U.S. Pat. No. 4,054,809 to Jefferies. The flux shield 28 is formed of a material such as copper or a copper alloy so that magnetic flux is concentrated in the shield 28, rather than in the flange 22. As a result, power losses in the machine can be significantly reduced, thereby increasing the overall efficiency of the machine and reducing temperatures within the sheets 14 at the ends of the stator 10. However, a drawback is that the flux shield 28 is heated by the eddy currents, resulting in heating of the shield 28 and heat transfer to the flange 22 by conduction and/or convection. The flux shield 28 also adds complexity and cost to the machine. Accordingly, it would be desirable if the flux shields 28 could be eliminated as separate discrete components of large dynamoelectric machines.